1. Field of the Invention
The present invention relates to a tilt/trim device for use with an outboard engine on a motorboat.
2. Description of the Relevant Art
Marine propulsion units for use on small marine vessels such as motorboats comprise an outboard engine, a stern bracket fixedly mounted on the transom of the motorboat hull, and a swivel bracket pivotally supported on the stern bracket, the outboard engine being mounted on the swivel bracket. The swivel bracket is vertically swung with respect to the stern bracket by a tilt/trim device. The outboard engine swings with the stern bracket so that the outboard engine can be lifted off the water (tilting action) and can be vertically angularly adjusted underwater (trimming action). The tilt/trim device can also perform the function of absorbing shocks produced when hit by driftwood or other foreign matter.
The tilt/trim device has at least one hydraulic cylinder/piston unit which can be extended or contracted when working oil stored in an oil tank is supplied into or discharged from a cylinder by an oil pump. One type of tilt/trim device has a cylinder/piston unit for effecting the tilting action and another cylinder/piston unit for effecting the trimming action. Another type of tilt/trim device has a single cylinder/piston unit for carrying out both the tilting action and the trimming action.
In the conventional tilt/trim device, if air is trapped in the tank, air bubbles are produced in the working oil in the tank due to vibrations caused by the outboard engine, and are drawn into the oil pump. Therefore, the air bubbles are introduced into the hydraulic circuit, which then fails to supply the working oil under predetermined pressure into the cylinder. When the hydraulic cylinder/piston unit is contracted, a volume of working oil which corresponds to the volume by which the piston enters the cylinder returns to the oil tank, thus increasing the amount of working oil in the oil tank. Therefore, the storage capacity of the oil tank should be large enough to store the necessary amount of working oil. With the large-size oil tank, however, the entire tilt/trim device is large in size.